linux-kernel - Re: [PATCH 1 of 4] mm: add a ptep_modify

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Date:	Wed, 18 Jun 2008 16:59:05 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	benh@...nel.crashing.org
CC:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	x86@...nel.org, xen-devel <xen-devel@...ts.xensource.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Hugh Dickins <hugh@...itas.com>,
	Zachary Amsden <zach@...are.com>,
	kvm-devel <kvm-devel@...ts.sourceforge.net>,
	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1 of 4] mm: add a ptep_modify_prot transaction	abstraction

Benjamin Herrenschmidt wrote:
> On Mon, 2008-06-16 at 04:30 -0700, Jeremy Fitzhardinge wrote:
>   
>> The only current user of this interface is mprotect
>>     
>
> Do you plan to use it with fork ultimately ?
>   

Good point, I'd overlooked that.  I guess that means using it in 
ptep_set_wrprotect().

At present the x86 ptep_set_wrprotect() just uses clear_bit on the pte, 
which is a locked cycle.  Is that significantly cheaper than an xchg + 
set?  (Same number of locked operations...)

    J
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